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If you stare at the Sun you’ll go blind. And if you spend a lot of time in the sun, you’re likely to get skin cancers. So visible light seems plenty dangerous to me.

Some of the damage may actually be from infrared and ultraviolet light, but these are close in frequency to visible light and very far from microwaves.

By the way, the intensity also matters, not just the frequency. In terms of photons, it matters not only how energetic each photon is, but also how many photons are arriving per second.

The dose (or, in this case, the intensity) makes the poison. You're constantly exposed to microwaves since that's what 99% of wireless communication devices use, and you're also constantly exposed to visible light unless you sleep in an isolation tank. Both can be dangerous if you increase the intensity sufficiently.

Assuming the intensity is the same, microwaves are more dangerous than visible light because they penetrate the skin to a greater depth (1-2 cm; more info is in Wikipedia).

Humans have more adaptation to visible light than to microwave radiation, because they were exposed to light for millions of years. This is expressed in two ways:

• Proteins in the epidermis (outer skin layer) are more resistant to heat than those in the deeper skin layers


• There are more nerve endings in the outer layers of skin, so dangerous heating by visible light causes more pain, urging you to escape the dangerous situation

Oh, and the most obvious difference: visible light is visible. Dangerous levels of visible light (e.g. in a solar cooker), to our eyes, look blinding and obviously dangerous. Dangerous levels of microwave radiation are invisible.

The danger of electromagnetic waves is a function of the photon energy, the intensity of the source and your distance from it, and the qualitative nature of the interaction of a specific frequency with organic matter.

That latter bit is very complex. The visible spectrum, down to about infra-red, doesn't penetrate the top layer of skin, or most clothing, so for the most part its interaction is limited to heating. Strong infrared can certainly cause burns. Strong visible light can certainly cause eye damage. But very high-intensity sources of visible light are rare in daily life, and very notably, we can see them, and avoid them.

Further down in the energy spectrum (longer wavelength) you get the "millimeter waves" of airport scanners, which can penetrate clothing but not skin, and then you're into the microwaves, uhf and vfh radio waves, and then the radio waves called shortwaves (high frequency), medium waves and longwaves (low frequency). Microwaves can penetrate into flesh, and radio frequencies can entirely traverse a human body, and these can cause very, very severe deep-tissue burns. Certain frequencies can also interfere with cardiac rhythm, which can be as fatal as it sounds.

Your home wifi equipment produces microwaves on pretty much the same frequencies as your microwave oven, but at milliwatt power and dispersed in all directions. The oven dumps hundreds of watts into a small enclosed space. That's the difference.

And we live in a sea of radio waves from microwave (cell phone, wifi) though uhf and vhf (two-way radios, broadcast TV and FM) and lower (broadcast radio). The key is the power. If you grab the antenna of your uncle's 500-W ham radio when he keys the mic, or climb the tower of a multi-kilowatt tv station, you'll get hurt, maybe very badly. But going about your normal business, you're probably absorbing less than a milliwatt of radio energy. And the only effect is heating, so it's little different than being in a room that's very slightly warmer.

Now, moving upward in energy from the visible spectrum, you get ultraviolet, x-rays and then a vast spectrum of increasing energy gamma. Not only can they penetrate into flesh, they have a very specific dirty trick: they have enough photon energy to ionize molecules, and when that happens to our dna and proteins, we start to have very bad days. This is a very specific capability that begins at a certain energy threshold.

Microwaves, as you've remarked, are in the opposite direction from UV, X-rays and gamma: lower photon energy, longer wavelength. They cannot duplicate the ionization danger of higher energies, no matter how intense their sources are.

Other answers already point out the matter of intensity. If you have a 1kW microwave that cooks your chicken and so call microwaves dangerous, you can equally cook chicken with 1kW visible light bulb. The difference is mostly how well and how deep the absorption goes, but the amount of energy is the same if the absorbed power is the same.

What makes the difference:

• Absorption coefficient; how WELL the light is absorbed. So, a black chicken will cook well in strong visible light, but a white chicken will require higher power, because it reflects more. Microwaves work well because they penetrate deeper before they get absorbed (due to longer wavelength), but also absorb WELL because the frequency is tuned near the resonance for water molecules.


• Resonances; If the wavelength matches exactly to one of the transition frequencies for molecules/atoms in the target, most of the energy is selectively absorbed just by those molecules. So, if you tuned your light specifically to a transition that breaks some specific bonds, or heat just specific tissues. This can do more damage because it may change chemistry, but luckily, breaking bonds requires quite high frequencies - see next case below. With MW and IR, you will still end up just heating the sample if you find a resonance (resonances in MW, IR and visible are mostly vibrational and rotational transitions, not bond changing, except for red-colored substances reacting to visible light, which you notice when red dyes bleach quickly in intense light).


• Ionization; If the energy of a SINGLE photon is enough to kick off an electron from a molecule/atom, then it's dangerous because it's actively affecting the chemistry (note that this is similar result than the resonant case above, but instead of having a precise frequency, it has way too much energy, with similar results). This is what is called ionising radiation (gamma/X-rays, down to UV range).

Note that resonance just means good absorbtion, nothing mystical. Water is mostly transparent for visible light because no significant vibrations of water molecule fall in this range - most are in IR and microwave, and there is another absorption range in UV.

Rule of thumb: microwaves, IR and visible light just heat you up. It's the heat is sufficient to raise the temperature into danger zone, it's dangerous, otherwise it's harmless. Only intensity matters (Watts per square meter), not the frequency.
Ionizing radiation (UV/X-ray/gamma) are dangerous because of chemical damage even at low intensity.

Microwaves are NOT ionizing radiation, so the wireless and mobile signals do absolutely nothing - the power is way too low, otherwise you'd need to charge your phone every 5 minutes.

It's more accurate to say microwave ovens are dangerous. Then again, so is visible light.

It's a question not of photon energy, but total energy. A typical microwave outputs on the order of 1 kilowatt of electromagnetic radiation which is almost entirely absorbed by the food within.

By comparison, the solar power at earth's surface, at maximum, is around 1 kilowatt per square meter. If it's cloudy, not at the equator, or not noon, it will be less. Most foods have a surface area of much less than a square meter, so the total electromagnetic radiation power received by something sitting in the sun is much less than a microwave oven.

For a fair comparison, what do you think would happen if a magnifying glass with an area of one square meter, on a very sunny day, were used to focus light on to something the size of what you'd put into a microwave oven?

There are a few more subtle differences. For example dangerous powers of visible light are so bright you'll surely close your eyes. Furthermore, visible light penetrates less deeply, so you're likely to feel the heat and move away before it does any more than superficial damage to your skin, like a sunburn. On the other hand, microwave radiation is invisible and penetrates more deeply, so you may suffer irreversible injury before even noticing the hazard. The cornea is especially prone to microwave injury since there's no protective reflex to protect it, it has little thermal mass and thus heats quickly, and there's little blood flow to cool it.

The trick with microwaves is that they use resonance frequencies of water. Some microwaves can be tuned to meat, vegetables or fish, since the resonance frequency can change slightly in composition. Visible light does not resonate with anything in our bodies.

Our generally photo-sensitive skin mostly reacts to UV-Ranges, even in a not yet ionizing spectrum. (The wavelength also determines how deep the light can enter the skin!)

With enough energy, visible light could be dangerous, but the energy needed is far greater without any resonance effects. (think of a child swing)

When we say that microwaves are less energetic, we are talking about the energy in a single photon. The number of photons is also important.

A single microwave photon is utterly harmless. Its only effect is heat, and it takes a serious amount of heat to damage us. But enough heat, in any form, will kill.

Visible light has enough energy that single photons can cause chemical reactions, but only in sensitive compounds. That is what happens in our eyes. The chemicals in our eyes are carefully constructed to be extra sensitive to light and that is what makes light visible.

Ultraviolet light is worse. Here the photons carry enough energy to cause unwanted chemical reactions in most organic compounds. Sun burn and skin cancer results.

Gamma ray photons from radioactivity are even worse, but they are fortunately rare.

There is nothing inherently more or less dangerous about microwaves. Yes, the type of damage via microwave vs. visible light vs. xray are different but weather or not the light cause damage has the same factor for all the spectrum of light - intensity.

Remember, modern computer-using humans constantly volunteer to be bather in microwaves. Wifi uses exactly the same frequency as microwave ovens. The difference between wifi and microwave ovens is the wattage - the amount of power used to generate the light - the intensity.

You can cook with visible light if you pump enough power into it - or somehow concentrate it. This is how sun ovens work and how you can burn paper with a magnifying glass. You can also cook with xrays if it is intense enough.

Side note: Most people don't realise this but high intensity infrared light can blind you as surely as staring at the sun. Just because it is invisible to your eyes does not mean the photons does not hit your retina. High intensity infrared spotlights are sold as part of security systems for infrared sensitive cameras (night vision).